HIGH STRENGTH STEEL KEEPS PASSENGERS IN PLACE
30 October 2002 - SSAB Tunnplåt AB

Designed for collision forces equal to six tonnes, but weighing only 16.4 kilograms.
That's the newly developed beam which keeps the centre seats in the new Volvo XC90 in place. Finnveden and Volvo Car have jointly met the requirements for low weight, a high level of safety, and passenger comfort with the help of a frame design in high strength steel.

The result has exceeded expectations, and the safety beam has now been nominated for the 2002 Swedish Steel Prize, the international engineering design accolade. The winner will be announced on 21 November at the prizegiving ceremony in Gothenburg.

The new Volvo XC90 has the interest of car purchasers the world over. Volvo's new SUV (sport utility vehicle) is packed with new technology and safety solutions which are intended to attract families, particularly in North America and Europe. An important new feature in the seven seater is the beam which secures the centre seat. A year of development work lies behind the technically complex design. The beam is positioned under the seat. In a collision, it is exposed to forces as high as six tonnes which it must absorb without exceeding a specified weight limit.

"It must be possible to fold the seatback forwards since the car is constructed with three rows of seats and we are therefore unable to secure the centre seat to the car sides," says Mats Bergman, technical director at Finnveden's Sheet Metal Components business area.

A complicating factor is the centre passenger's seat belt housing, which is fitted to the back section. There are no side attachments, all of the collision forces must consequently be absorbed by the beam in the floor.

The project was assigned to Finnveden in spring 2000. The prototype beam used by the company weighed 22 kilograms. It failed to meet all of Volvo's requirements, which included maximum post-collision bending of 20 millimetres in the rear beam. The beam was also too heavy, the weight limit being 20 kilograms. Selecting thicker steel beams was therefore not an option. Engineers at Finnveden and Volvo instead commenced work on a completely new design in high strength steel.

"By using high strength steel we were able to remove a lot of material, thereby reducing the weight," Mr Bergman adds. "After a lot of hard work we arrived at a design which was almost 25 per cent lighter. The 16.4 kilogram beam easily met the exacting safety requirements."

"It took us more than a year to complete the work on the final version of the frame," says Martin Wallström, development engineer at Volvo Car and manager of the beam project.

"We must ensure that we develop cars which are world class with regard to safety and quality. This involves time-consuming development work trying out new solutions and materials. However, we have never considered using anything apart from steel. For us, it has proved unbeatable as a design material for parts where safety, strength and price are decisive factors.

"The new frame consists of 16 different parts which are welded together at Finnveden's production plant in Olofström in the south of Sweden. The two long transverse beams are already rolled and cut when Finnveden receives them. The open-type front transverse beam is in ultra high strength steel and has a tensile strength of 1200 N/mm2. The closed-type rear transverse beam is manufactured in hardened boron steel, which has even higher tensile strength. The rear beam is rolled and hardened in the same process, a patented production method carried out by Accra Teknik in Piteå in the north of Sweden. The two side beams, which form the design's collision boxes, are pressed from high strength steel and have a tensile strength of 600 N/mm2.

"Careful thought has been given to all the parts in both the product and the production process," says Mr Bergman. "We can see that our well-targeted research work has borne fruit. The new frame is a solution which is cost-efficient, secure and eco-friendly."

Finnveden and Volvo Car have been jointly nominated for the 2002 Swedish Steel Prize,the international engineering design accolade, for their safety frame. The winner will be announced from the four finalists at the prizegiving ceremony which will be held in Gothenburg on 21st November. The Swedish Steel Prize was instituted by SSAB Swedish Steel.

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